1. Field of the Invention
This invention relates to load positioners used in automatic production machinery and specifically to apparatus for positioning sawmill lumber manufacturing machine elements such as saws, fences, and guides.
2. Description of the Prior Art
In a sawmill, the means most often used for controllably and repeatably positioning saws, fences, guides, alignment lights, and so on, is the binary stack of bottoming pneumatic cylinders. For example, the series head-to-toe stack of four cylinders having individual throws of 1/8", 1/4", 1/2", and 1", would be capable of setting to within 1/8" of any position within a 17/8" total range.
By adding more cylinders to the stack, the total range may be extended, or the resolution may be improved, at least in theory. Generally it has been impractical to use cylinders with throw shorter than 1/16".
A digital electronic circuit typically is used to generate from a single operator switch closure, the multitude of electrical signals--two per cylinder--required to command the valves that in turn control extension and retraction of individual cylinders. At the instant that a broadside set command is applied to the valves, the net transient motion produced can be momentarily directed away from the new target instead of toward it, if a long cylinder in the stack is faster setting than one or more of the shorter cylinders in the stack. In some applications, such erratic or false motion is not tolerable.
The practical limit of resolution experienced with bottoming cylinders has been about 1/16". The repeatable realization of this resolution requires continuous attention to the tightness of the many mechanical connections in the stack, and also to elimination of the tendency toward buckling of the long stack under load.
More recently, highly accurate setworks have been fabricated using a single hydraulic cylinder in place of the long stack of air cylinders. The single cylinder is controlled by a proportional electrically controlled valve, that is, a valve though which the flow of hydraulic oil is proportional to the electric current through a coil which is a part of the valve mechanism. The current through the coil is in turn controlled by a computer which constantly tracks the position of the object whose position is being controlled, and makes appropriate decisions as to the magnitude and direction of coil current to be applied.
The people at Elworthy Company in Vancouver BC in Canada, manufacture a setworks that is based on moving a magnet past a sensor. Their sensor is a sensitive reed switch, and the axis of their magnet is at right angles to the direction of motion. Their setworks, called the "Dynaset," also uses a miniature stack of bottoming cylinders to position the sensor--not the load--and in practice it achieves repeatable sets to within 1/16" to 1/8". The Dynaset uses an on-off type of hydraulic valve to control a single hydraulic cylinder. It is not capable of operating a proportional valve.
The present practice of skilled setworks designers is to use a single hydraulic cylinder as primemover under control of a proportional servo valve such as a MOOG model 62, which is in turn under control of a microcomputer; with load position being sensed by a digital rotary or linear incremental encoder, or by an absolute position transducer of the echo-ranging type. The incremental encoder offers greater intrinsic accuracy independent of range, while the absolute transducer offers greater immunity to the practical problems associated with noise and referencing. Although resolution of 0.010" is in practice adequate for sawmill positioners, marketing considerations have caused designers currently to strive to exhibit a resolution of 0.001".
When using an incremental encoder, the computer or an auxilary circuit must count the pulses generated to keep track of the position of the load. The maximum speed with which the cylinder may be moved must be limited such that the pulse rate generated has a maximum frequency sufficiently lower than the frequency of ambient electrical noise, so that such noise can be identified and ignored.
When using echo-ranging, unless target overshoot is acceptable, the cylinder slew speed must again be limited such that the average of, for example, the most recent ten soundings is a sufficiently accurate representation of the actual instantaneous load position. With a Temposonic transducer, for example, the averaging technique is essential if resolution better than about 0.010" is required over a 24" range. Thus each technique of the current practice involves cylinder slew speed limitations.
The microcomputer offers the designer and the user a range of flexibility that is in principle nearly unlimited. For example, a microcomputer based setworks will in principle be capable of positioning the load anywhere within the range of its cylinder to within 0.010" of an arbitrary target. Flexibility is however a two-sided feature. Greater flexibility is inevitably accompanied by greater probability of miss-sets. In other words, the reliability of the machine is reduced by its flexibility.
In summary, the current state of the art in sawmill setworks is well represented by the single hydraulic cylinder with built-in absolute position transducer, or built-in incremental encoder, coupled with a microcomputer acting as controller. Examples of absolute transducer types are the MOOG A86 Servo-actuators, and the "Accu-Set" cylinder manufactured by United Machine and Control. An example of the incremental type is the "Inovec" manufactured by Fluid Air Components, Inc.
Microcomputer based setworks are expensive to program, require setup and maintenance skills not generally found in sawmills, and intrinsically exhibit failure modes avoidable by simpler, less flexible equipment.
3. Objects of the Invention
One purpose of the present invention is to secure the advantages of the single hydraulic cylinder operating as primemover, and the accuracy realizeable with the proportional valve operating in a self-regulating closed loop, while eliminating the computer.
Another purpose of the present invention is to eliminate the limitation of coarse resolution that is inherent in the use of bottoming cylinders, and while retaining comparable simplicity, achieve continuous setability, or in other words, infinite resolution in principle.
Another purpose of this invention is to provide a setworks that can be set up for use and also have its "sets" modified or adjusted by a person using basic mechanical skills as opposed to requiring attention of a person having electronic or computer related skills.
Another object of my invention is to provide a setworks whose accuracy is independent of slew speed.
Another object of the present invention is to provide a setworks of reduced complexity and to thereby secure improved reliability.
Another object of my invention is to provide a basic method of magnetic control signal creation that can be applied under a variety of conditions to achieve simplified solutions to the problems of automatic and manual positioning of production machinery, as will be more evident in the following specification.